This invention relates in general to lithographic imaging members suitable for on- or off-press imaging, and particularly to waterless imaging members that require no wet processing or wiping after imaging. This invention also relates to a method of imaging such imaging member using for example digital means and to a method of printing with the imaged members.
Very common lithographic printing plates include a metal or polymeric support having thereon an imaging layer that is sensitive to visible or UV light. Both positive- and negative-working printing plates can be prepared in this fashion. After exposure to a light source, and possibly to a heat source, either imaged or non-imaged areas are removed using wet processing chemistries.
Thermally sensitive printing plates are becoming more common and are available at least from Kodak Polychrome Graphics. Such imaging members include an imaging layer comprising a mixture of dissolvable polymers and an infrared radiation-absorbing compound. While these imaging members can be imaged using digital means (such as lasers) and can be utilized in what is known as xe2x80x9ccomputer-to-pressxe2x80x9d imaging systems, they still require post-imaging wet processing using alkaline developer solutions.
Dry planography or waterless printing is well known in the art of lithographic offset printing and has several advantages over conventional offset printing. Dry planography is particularly advantageous for short run and on-press applications. It simplifies press design by eliminating the fountain solution and aqueous delivery train. Careful ink water balance is unnecessary, thus reducing rollup time and material waste.
An unexposed waterless printing plate typically comprises a layer of ink repellent material over a layer of ink accepting material or an ink-accepting surface. Because of their low surface energies and their ability to swell in the long-chain alkane solvents used in printing inks, silicone rubbers, such as poly(dimethylsiloxane) (identified herein as xe2x80x9cPDMSxe2x80x9d) and other derivatives of poly(siloxanes), have long been recognized as preferred waterless-ink repelling materials. Preparation of the printing plates involves the imagewise removal of the ink repellent silicone rubber to expose the underlying ink accepting material or surface.
Various methods of removing the silicone rubber layer have been developed. Imaging of dry planographic printing plates with infrared lasers has been described in Canadian Patent 1,050,805 (Eames) and by Nechiporenko and Markova, xe2x80x9cAdvances in Printing Science and Technology,xe2x80x9d Proceedings of the 15th International Conference of Printing Research Institutes, June 1979, Pentech Press, London, pp. 139-148. The silicone rubber layer is coated over a heat-absorbing layer containing an infrared absorbing material in nitrocellulose. Imagewise exposure with an infrared laser partially disrupts the heat-absorbing layer, allowing it and the overlying silicone layer to be removed from the exposed regions with a solvent.
Infrared imaging of printing plates with xe2x80x9cablatablexe2x80x9d layers has also been described in U.S. Pat. No. 4,718,340 (Love III), WO 92/07716 (Landsman), WO 94/18005 (Verburgh et al), U.S. Pat. No. 5,379,698 (Nowak et al), U.S. Pat. No. 5,310,869 (Lewis), U.S. Pat. No. 5,339,737 (Lewis et al), U.S. Pat. No. 5,385,092 (Lewis et al), U.S. Pat. No. 5,351,617 (Williams), U.S. Pat. No. 5,353,705 (Lewis et al) and U.S. Pat. No. 5,487,338 (Lewis). These documents describe the use of direct digital imaging on-press or a platesetter.
In each of these methods, mechanical wiping or washing with liquids is required to remove the silicone rubber debris clinging to the plate after exposure. This problem arises because of the conflicting needs to have wear-resistant silicone layers for long press runs while maintaining the ease of layer removal during thermal imaging. Wiping has several drawbacks. It is difficult to reproducibly remove all stray material with automated cleaning stations. Moreover, wiping can scratch or abrade the printing plate.
A truly processless printing plate, that is one that does not require a separate processing step to remove the silicone rubber debris after imaging, would have several advantages. The post-imaging development or wiping step would be eliminated, simplifying the process for preparing the printing plate. In addition, any scratching or abrading of the plate surface caused by development would be eliminated. If desired, the plate could be exposed on the printing press, eliminating any potential damage to the plate caused by handling and mounting on the press after imaging.
There are three key requirements for an ink repellent polymer to be useful for a thermally imageable processless printing plate that is imaged using ablation. The ink repellent polymer must form a solid film at room temperature to resist damage from the press. It also must release ink, and must be easily removed by the imaging step alone or by the normal action of the press after imaging.
U.S. Ser. No. 08/749,050 (filed Nov. 14, 1996 by Bailey et al) discloses a class of silicone copolymers that exhibit these desirable attributes. The plates prepared using those copolymers can be imaged and used to print many thousands of impressions. Unfortunately such printing plates still suffer from the conflicting need to be durable on press but readily thermally imaged without the need for wiping or washing. Optimum exposure for ablation plates is therefore relatively high, leading to undesirable system costs in power and time.
One approach toward processless, non-ablation waterless printing plates involves the use of xe2x80x9cswitchablexe2x80x9d polymers. These polymers will undergo thermally driven chemical reactions in which ink accepting or rejecting characteristics are either created or destroyed under imaging conditions. In addition to not needed wet processing, such plates have the advantage of not needing any type of material collection devices that ablation-based plates require.
Although a number of switchable polymer-based printing plates are known, there remain technical barriers towards the utilization of this technology in commercially feasible products. Three difficulties commonly experienced in the design switchable polymer-based plates are physical wear of the plates and the related problems of background toning and blanket toning.
In switchable polymer-based printing plates, a major challenge lies in the creation of a thin synthetic polymer surface that has both adequate physical toughness and resistance to toning. A need exists in the industry for a thermally imageable processless and waterless imaging member in which the ink repellent surface layer is a polymer that is wear resistant and readily imaged by xe2x80x9cswitchingxe2x80x9d without the problems usually encountered with prior art materials.
The problems noted above have been overcome with a thermal imaging member comprising a support having thereon:
(a) an ink-repellant subbing layer that is swellable in waterless ink solvents, and
(b) an ink-repellant, thermally sensitive imaging layer that comprises a photothermal conversion material and a thermally sensitive copolymer comprising one or more silicone segments and one or more thermally sensitive xe2x80x9chardxe2x80x9d segments, the silicone segments comprising from about 50 to about 99 weight % of the copolymer, the imaging layer being capable of becoming ink-accepting upon exposure to thermal energy.
This invention also provide a method of imaging comprising:
A) providing the imaging member described above, and
B) imagewise exposing the imaging layer of the imaging member to thermal energy to provide exposed and unexposed areas in the imaging layer whereby the exposed areas are rendered more oleophilic than the unexposed areas by the thermal energy.
Further, this invention provides a method of printing comprising steps A and B noted above and:
C) inking the imaging layer and imagewise transferring the ink to a receiving material.
The imaging members of this invention provide several advantages. They require relatively low thermal exposure during imaging. Furthermore, nothing must be removed from the imaging member because the surface layer xe2x80x9cswitchesxe2x80x9d in its affinity to ink solvents and the ablation processes are avoided. As a result, the imaging method does not require a wiping step or washing with liquids. Thus, the imaging members can be directly imaged using digital information supplied for example using a laser. They have high writing sensitivity, high image quality, short roll up time and long run length.
The surface imaging layer includes a thermally sensitive copolymer having silicone segments and thermally sensitive xe2x80x9chardxe2x80x9d segments. These xe2x80x9chardxe2x80x9d segments provide physical integrity and thermal sensitivity while the silicone segments provide ink releaseability. The balancing of relative amounts of these segments provides all of the desired properties for the imaging layer.
While the surface layer is relatively thin, it is also highly durable after imaging and highly sensitive to the thermal energy so that the exposed areas become more oleophilic and ink accepting during imaging. The surface imaging layer provides a good balance of physical toughness and resistance to scumming and blanket toning while having the desired thermal sensitivity. The thermally sensitive copolymer can be formulated with a suitable photothermal conversion material to increase thermal sensitivity.